1. Field of the Invention
The present invention relates to a thermostatic expansion valve which is used in the refrigeration apparatus and is provided with a diaphragm serving to receive at one side surface thereof the pressure of an operating fluid fed from a thermal bulb located at the outlet of an evaporator of the refrigeration apparatus, the other side surface of the diaphragm being connected to a valve mechanism, wherein when the diaphragm is displaced responsive to any change in the pressure of the operating fluid, the valve mechanism is controlled in accordance with a change in a difference between the pressure applied to the one and the other side surfaces of the diaphragm, by the displacement of the diaphragm produced by the change in the pressure difference, to control the flow rate of refrigerant flowing into the evaporator. More particularly, it relates to a thermostatic expansion valve which has a valve housing supporting the diaphragm and having a refrigerant passage and a valve seat formed in the refrigerant passage, the refrigerant passage being connected at its one end to the inlet of the evaporator and being supplied at its other end with a highly pressurized fluid flowing from a compressor of the refrigeration apparatus, the valve seat being controlled the valve opening thereof by a valve body of the valve mechanism, and the valve housing further comprising an equalizing-signal passage for introducing a pressure-reduced refrigerant, which is reduced in pressure by a co-operation between the valve body of the valve mechanism and the valve seat of the valve housing, from a pressure-reduced-refrigerant outlet space located at the downstream side of the valve seat in the refrigerant passage to the other side surface of the diaphragm.
The thermostatic expansion valve of this type was conventionally used, for example, in a refrigeration apparatus for an air-conditioner of a small-sized car. This kind of thermostatic expansion valve is simpler in construction, easy to assemble, and lower in cost, as compared with those conventional ones which use an equalizing pipe instead of the equalizing-signal passage to introduce the superheated refrigerant vapor from the outlet of the evaporator to the other side surface of the diaphragm.
In a case that the difference between the superheated vapor pressure of the refrigerant at the outlet of the evaporator and the pressure of the refrigerant at the inlet of the evaporator is small due to that the flow rate of the refrigerant is small in the refrigeration apparatus and the pressure loss of the refrigerant is small in the evaporator, the simply constructed thermostatic expansion valve can appropriately control the flow rate of the refrigerant to the evaporator without causing any larger error as compared with the above-mentioned conventional thermostatic expansion valve of the other type which uses the superheated vapor pressure of the refrigerant at the outlet of the evaporator. Namely, the flow rate of the refrigerant flowing to the inlet of the evaporator can be controlled in such a way that the degree of the superheat of the refrigerant at the outlet of the evaporator keeps at a predetermined value to enable the refrigeration apparatus to achieve the optimum refrigerating efficiency.
In this simply constructed thermostatic expansion valve, however, the flow rate of the refrigerant supplied to the evaporator is smaller than that in the case of the above-mentioned conventional thermostatic expansion valve of the other type which use the superheated vapor pressure of the refrigerant at the outlet of the evaporator, when the difference between the superheated vapor pressure of the refrigerant at the outlet of the evaporator and the pressure thereof at the inlet of the evaporator becomes large due to that the flow rate of the refrigerant in the refrigeration apparatus becomes large and the pressure loss in the evaporator becomes large. More specifically, the degree of superheat of the refrigerant at the outlet of the evaporator becomes higher than the optimum value at the time when the refrigeration apparatus achieves the optimum refrigerating efficiency. This prevents the refrigeration apparatus from being operated at its optimum refrigerating efficiency.
U.S. Pat. No. 4,342,421 discloses another well-known thermostatic expansion valve to eliminate the above-mentioned drawback caused in the case that the flow rate of the refrigerant in the refrigeration apparatus becomes large in the above mentioned simply constructed thermostatic expansion valve. The construction of the expansion valve of USP is schematically shown in FIG. 1.
This thermostatic expansion valve, similar to the ones of the same type to which no improvement was added, has right-angled "T" shaped valve housing 10 and diaphragm 12 is housed in a sealed space in the upper extended end portion of valve housing 10. Thermal bulb 14 is connected to the upper extended end portion of valve housing 10 at the upper side of diaphragm 12. Valve-body driving pin 16 of a valve mechanism connected to the undersurface of diaphragm 12 extends toward the lower extended end portion of valve housing 10 in valve housing 10. Refrigerant passage 18 extends like a reversed "L" shape from the lower extended end surface of the lower extended end portion to the side extended end surface of the side extended end portion, and valve seat 20 of the valve mechanism is mounted at the inner end of lower extended area 18a of refrigerant passage 18. The lower end of valve-body driving pin 16 is inserted into a center hole of valve seat 20, and contacts valve body 24 of the valve mechanism which is located at the lower extended area 18a of refrigerant passage 18 and is urged by urging means 22 to sit on valve seat 20 from below.
Lower extended area 18a of refrigerant passage 18 is connected to a compressor (not shown) through a condenser (not shown) and is supplied with highly pressurized refrigerant from the compressor. The highly pressurized refrigerant in lower extended area 18a is reduced in pressure while passing through valve seat 20 to enter into side extended area 18b of refrigerant passage 18, and the pressure-reduced refrigerant further passes through side extended area 18b and flows to the inlet of an evaporator (not shown) connected to side extended area 18b. Side extended area 18b which serves as a pressure-reduced refrigerant outlet space in this manner is communicated with a diaphragm-sealed space on the other side of diaphragm 12 through equalizing signal passage 26 extending through the upper extended end portion of housing 10.
The thermostatic expansion valve disclosed in the U.S. Patent is different from the no-improved ones of the same type in that side extended area 18b serves as a venturi tube. More specifically, side extended area 18b is a stepped hole having a small-diameter portion located more inside the opening of equalizing signal passage 26 and a large-diameter portion located more outside the opening of equalizing signal passage 26. Ring-shaped cylinder 28 is formed on the stepped portion of area 18b to coaxially project toward the large-diameter portion of area 18b. The outer circumferential surface of cylinder 28 radially faces the opening of equalizing signal passage 26, and equalizing signal passage 26 is communicated with the outer large-diameter portion of side extended area 18b through a ring-shaped clearance formed between the inner circumferential surface of the outer large-diameter portion and the outer circumferential surface of cylinder 28.
In this case, when the pressure-reduced refrigerant which is reduced in pressure by flowing from lower extended area 18a into valve seat 20 toward side extended area 18b flows from the inner small-diameter portion surrounded by cylinder 28 to the outer large-diameter portion at side extended area 18b, the velocity of the refrigerant becomes faster at the vicinity of the projected end of cylinder 28 than in the center of cylinder 28. As the result, pressure in equalizing signal passage 26 becomes lower as the flow rate of the pressure-reduced refrigerant flowing into side extended area 18b becomes larger.
This decrease of pressure in equalizing signal passage 26 which is loaded on diaphragm 12 instead of the superheated vapor pressure of the refrigerant at the outlet of the evaporator can compensate the increase of the difference between the superheated vapor pressure of the refrigerant at the outlet of the evaporator and the pressure of the refrigerant at the inlet of the evaporator, said increase of the difference being caused as the flow rate of the pressure-reduced refrigerant flowing into side extended area 18 becomes larger. Namely, the difference between the superheated vapor pressure of the refrigerant at the outlet of the evaporator and the pressure of the refrigerant at the inlet of the evaporator, which is caused by the increase of pressure loss of the refrigerant in the evaporator resulting from the increase of the flow rate of the refrigerant, can be made zero by this decrease of pressure in equalizing hole 26 at the inlet of the evaporator. Therefore, the flow rate of the refrigerant flowing to the inlet of the evaporator can be controlled to keep the degree of the superheat of the superheated refrigerant vapor at the outlet of the evaporator constant and to enable the refrigeration apparatus to achieve the optimum refrigerating efficiency.
However, the thermostatic expansion valve disclosed in the U.S. Patent makes it difficult to work side extended area 18b of refrigerant passage 18 where cylinder 28 is formed, and also to work equalizing signal passage 26 which faces the outer circumferential surface of cylinder 28. Further, the ring-shaped clearance, formed between the inner circumferential surface of the outer large-diameter portion of side extended area 18b of refrigerant passage 18 at which equalizing signal passage 26 is opened and the outer circumferential surface of cylinder 28, makes the flow of the pressure-reduced refrigerant at side extended area 18b being turbulent to make noises. In addition, this turbulent flow suddenly changes the pressure in equalizing signal passage 26 for a short time period, thereby changing for a short time period the controlling of the flow rate of the refrigerant flowing to the inlet of the evaporator.